According to the Special Theory of Relativity, the velocity of a moving body is always less than the velocity of light. Since the energy of motion of… - Gerald James Whitrow

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According to the Special Theory of Relativity, the velocity of a moving body is always less than the velocity of light. Since the energy of motion of a body depends on its inertial mass and its velocity, it follows that if the energy of a body is increased indefinitely by the continual application of a force, the inertial mass of the body must be increased too; for, if not, the velocity would ultimately increase indefinitely and exceed the velocity of light. Einstein found that, corresponding to any increase in the energy content of a body, there is an equivalent increase in its inertial mass. Mass and energy thus appeared to be different names for the same thing, the energy associated with a mass M being Mc², where c is the velocity of light; and the mass M of a body moving with velocity v he found to be given by the following formula<math>M = \frac {m}{\sqrt{(1 - \frac {v^2}{c^2}}}</math>